Liquid crystal display elements are advantaged over other display elements in terms of its thin thickness, light weight, and low power consumption. The liquid crystal display elements are widely used in image display apparatuses such as televisions, video cassette recorders, and the like, and OA (Office Automation) apparatuses such as monitors, word processors, personal computers, and the like.
Conventionally known liquid crystal display methods of the liquid crystal display elements include, for example, a TN (Twisted Nematic) mode in which a nematic liquid crystal is used, display modes in which FLC (Ferroelectric Liquid crystal) or AFLC (Antiferroelectric Liquid crystal) is used, a polymer dispersion type liquid crystal display mode, and other modes.
Among the liquid crystal display methods, for example, the TN (Twisted Nematic) mode in which the nematic liquid crystal is used is conventionally adopted in the liquid crystal display elements in practical use. The liquid crystal display elements using the TN mode have disadvantages of slow response, narrow viewing angle, and the other drawbacks. Those disadvantages are large hindrances for the TN mode to take over CRT (Cathode Ray Tube).
Moreover, the display modes in which the FLC or AFLC is used, are advantageous in their fast response and wide viewing angles, but significantly poor in anti-shock property and temperature characteristics. Therefore, the display modes in which the FLC or AFLC is used have not been widely used.
Further, the polymer dispersion type liquid crystal display mode, which utilizes scattering of light, does not need a polarizer and is capable of providing a very bright display. However, in principle, the polymer dispersion type liquid crystal display mode cannot control the viewing angle by using a phase plate (retardation film). Further, the polymer dispersion type liquid crystal display mode has a problem in terms of its response property. Thus, the polymer dispersion type liquid crystal display mode is generally not as advantageous as the TN mode.
In all the foregoing display methods, liquid crystal molecules are oriented in a certain direction, and thus a displayed image looks differently depending on an angle between a line of vision and the liquid crystal molecules. On this account, all these display methods have viewing angle limits. Moreover, all the display methods utilize rotation of the liquid crystal molecules, the rotation caused by application of an electric field on the liquid crystal molecules. Because the liquid crystal molecules are rotated in alignment all together, responses take time in all the display methods. The display modes in which the FLC and the AFLC are used[ ] are advantageous in terms of response speed and viewing angle, but have a problem in that their alignment can be irreversibly destroyed by an external force.
In contrast to those display methods in which rotation of molecules by the application of the electric field is utilized, there is also a display method in which the secondary electro-optical effect is utilized.
The electro-optical effect is a phenomenon in which a refractive index of a material is changed by an external electric field. There are two types of electro-optical effects: one is an effect proportional to the electric field and the other is an effect proportional to the square of the electric field. The former is called the Pockels effect; the latter is called the Kerr effect. The Kerr effect was adopted early on in high-speed optical shutters, and has been practically used in special measurement instruments. The Kerr effect was discovered by J. Kerr in 1875. So far, organic liquids such as nitrobenzene, carbon disulfide, and the like, are known as materials showing the Kerr effect. These materials are used, for example, in the aforementioned optical shutters, and the similar devices. Further, these materials are used, e.g., for measuring the strength of high electric fields for power cables and the like, and similar uses.
Later on, it was found that liquid crystal materials have a large Kerr constant. Research has been conducted to utilize the large Kerr constant of the liquid crystal materials for use in light modulation devices, light deflection devices, and optical integrated circuits. It has been reported that some liquid crystal compounds have a Kerr constant more than 200 times higher than that of nitrobenzene.
Under these circumstances, studies for using the Kerr effect in display apparatuses has begun. It is expected that use of the Kerr effect attains a relatively low voltage driving because the Kerr effect is proportional to the square of the electric field. Further, it is expected that the utilization of the Kerr effect attains a high-response display apparatus (because, e.g., the Kerr effect shows a response property of several μ seconds to several m seconds, as its basic nature).
A significant practical problem to be overcome for utilizing the Kerr effect in display elements is that utilization of the Kerr effect requires a larger driving voltage compared with conventional liquid crystal display elements. To solve this problem, Publication of Japanese Patent Application, publication No. 2001-249363 (Tokukai 2001-249363; published on Sep. 14, 2001) (hereinafter, referred to as Patent publication 1) teaches a display element in which orientation of negative type liquid crystalline molecules is carried out, with substrates having a surface that have been subjected to alignment treatment, in order that the Kerr effect may be easily generated in the display element.
In the display element described in Patent Document 1, negative type liquid crystalline molecules are provided between a pair of substrates. Here, the wording “negative type” indicates that the liquid crystalline molecules show negative dielectric anisotropy. Moreover, electrodes are provided respectively on inner sides of the substrates. Alignment films which have been treated with rubbing process are provided on the electrodes. Moreover, on outer sides of the substrates, polarizers are so provided that their absorption axes cross each other perpendicularly. Moreover, rubbing directions of the alignment films provided on the electrodes are parallel and directed in the same or opposite directions. Further, the rubbing directions make 45 degrees with the absorption axes of the polarizers.
In the display element of Patent Document 1, having the above arrangement, an electric field (voltage) is applied between the electrodes so as to generate the electric field along a normal direction of the substrates. When the electric field is applied, the polarized negative type liquid crystalline molecules are oriented along an electric field direction in such a manner that the molecules are so directed that their major axial directions are parallel to the rubbing direction (the electric field direction is a direction in which an electric field is applied). With this arrangement, the display element of Patent Document 1 attains an optical response property in which its transmittance is increased by the electric field (voltage) application.
However, the art disclosed in Patent Document 1 has a problem in that the region in which the Kerr effect can be easily generated is limited to a vicinity of surfaces of the substrates. More specifically, in the art of Patent Document 1, only the molecules near the aligned surface of the substrates can be oriented. Therefore, the art of Patent Document 1 provides only little reduction of the driving voltage.
The small reduction in driving voltage occurs because the molecular orientation caused by the electric field application, i.e., the molecular orientation caused by the Kerr effect, has a short long-range order. That is, for example, in liquid crystal display devices such as those of the TN mode, orientational directions of the liquid crystal molecules are changed along substantially a whole range in the normal direction of the substrates, whereas in the liquid crystal display device using the Kerr effect, it is difficult to pass on molecular orientational order occurred in the vicinity of the substrates to a cell interior (bulk region). Because of this, the art of Patent Document 1 cannot significantly reduce the driving voltage to overcome the practical problem.
Further, in applying the art of Patent Document 1 in the display element in which the negative type liquid crystalline molecules are oriented by generation of the electric field along the normal direction of the substrates, there is a problem in that the major axes of the molecules in the bulk region are not aligned in one direction. In the vicinity of the rubbed surfaces (alignment films) of substrates, the electric field application causes the liquid crystal molecules to be oriented along the rubbing direction, whereas in the bulk region far from the substrates, the major axes of the molecules are directed randomly in all directions in a plane of the substrates. This is because the polarization of the molecules exist substantially along a minor axial direction of the molecules even though the polarization of the molecule are oriented. That is, even if the polarization is oriented (that is, orientational polarization occurs) by the electric field application, the bulk region is optically isotropic when viewed from a front direction (a direction along the normal direction of the substrates). Thus, the bulk region makes no contribution to the optical response.
Therefore, even if the art of Patent Document 1 is applied in a display element, a practical level of applied voltage can cause the optical response only in the vicinity of the substrates. A driving voltage much higher than a practical level is required to cause the optical response in the bulk regions.
Moreover, the display element disclosed in Patent Document 1 has a problem in that light leakage occurs when no electric field is applied. This light leakage causes low contrast. Further, the display element disclosed in Patent Document 1 has a problem in that a coloring phenomenon (e.g., a displayed image appears yellowish when the electric field is applied) occurs.